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Bearing in mind the various uncertainties involved in performing slope stability analyses, and in particular the determination of the shear strength parameters and the pore water pressure, there seems little point, so far as the determination of the value of the factor of safety is concerned, in trying many different assumptions regarding the slope of the inter-slice forces. A reasonably reliable value for the factor of safety can be obtained by assuming the inter-slice forces to be parallel (Spencer, 1973).

However, if the line of thrust is unreasonable (far outside the middle third of the inter-slice) it may be considered worthwhile obtaining solutions using other assumptions regarding the slope of the inter-slice forces, in which case the following recommendations are made. The inter-slice forces may be assumed to be parallel, that is f(x)=1, except at the upper end of the slip surface where their slope should be reduced, that is f(x) becoming less than one. Such a distribution of f(x) is shown in Figure 12. It should be pointed out that it is the shape of the distribution curve rather than its magnitude which determines the result The addition of a tension crack can also create a more reasonable line of thrust at times.

Another instance in which it may be necessary to use non-parallel inter-slice forces is when the solution fails to converge using parallel inter-slice forces. However, this problem can usually be solved by changing the initial estimates of F and .

Figure 12 Typical distribution of f(x) for non-parallel side forces.

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